I'm gonna guess a generic "Ranger" class because we've already had Rogue/Amazon/Druid and they need a ranged fighter and a nature-magic user, and "Ranger" covers both of those. Actually, I have a theory they're gonna have 6 classes in the game because at this point the game is mostly likely not coming out until 2011, which gives them more time, and I'm sure they have too many ideas to fit into 5 classes.
They are probably going to have 7 classes, like last time, with two being part of the expansion.
There's no reason for the monk to not be the range character.
Not really. Witch Doctor fits in the diablo setting pretty easy and I thought the WD was awesome from day one. The only people complaining where those who wanted the Necro and felt threated that the class was replaced with the WD.
SO far all the classes have felt like natural fits to the game... except the Monk.
Seriously, there was so much to pick from, and they went Monk.